1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to the structure for preventing the intrusion of a foreign material such as dust which intrudes into a backlight portion arranged inside a liquid crystal display panel.
2. Background Art
A liquid crystal display device is, basically, an image display device which is configured such that liquid crystal is filled between two substrates consisting of a first substrate and a second substrate preferably formed of a glass substrate, and an image is displayed by making use of changing of the alignment direction of the liquid crystal corresponding to an electric field applied to the liquid crystal from pixel selection electrodes formed on the substrate. A currently most-popularly-used all transmissive liquid crystal display device adopts the structure where light which is projected from a backlight (light source) mounted on a back surface of a liquid crystal display element is polarized by approximately 90 degrees by a liquid crystal layer and is allowed to pass through a polarizer thus converting an electronic latent image into a visible image which can be observed with naked eyes.
In an image display device which uses a non-light-emitting type liquid crystal display panel, an electronic latent image formed on the liquid crystal display panel is visualized using an external illumination means. The external illumination means is constituted by arranging a lighting device on a back surface or a front surface of a liquid crystal display panel except for the structure which makes use of a natural light. Particularly, in a display device which required high brightness, the structure which arranges a lighting device on a back surface of a liquid crystal display panel constitutes a main stream. This lighting device is referred to as a backlight.
The backlight is roughly classified into a side-edge-type backlight and a direct-type backlight. The side-edge-type backlight adopts the structure where a linear light source represented by a cold cathode ray tube is arranged along a side edge portion of a light guide plate formed of a transparent plate, and is popularly used as a display device which is requested to satisfy a demand of the reduction of thickness of a display device such as a personal computer. On the other hand, in a large-sized liquid crystal display device such as a display device used in a display monitor or a television receiver set, a direct-type backlight is popularly used. The direct-type backlight adopts the structure where a light source is arranged directly below a back surface side of a liquid crystal display panel.
In the liquid crystal display device, a liquid crystal display panel and a backlight are integrally formed for realizing the narrow picture frame and the reduction of weight of the liquid crystal display device. The liquid crystal display panel is held and fixed by being housed in a frame-shaped metal frame which is formed of a metal-plate molded body. The backlight adopts the structure where the backlight is mechanically fixed to a frame-shaped mold casing by fitting engagement or using bolts.
This type of liquid crystal display device is used, for example, as a monitor of medical equipment which is formed by assembling a liquid crystal display panel and a backlight in a casing. In the monitor, a plurality of air-cooling fans is arranged in the inside of the casing. These fans are used as an outside air suction fan and an air exhaust fan for cooling the inside of the monitor. Conventionally, when such a cooling fan is operated, a foreign material such as dust floating outside is sucked into the inside of the monitor and the foreign material is circulated in the inside of the monitor. Accordingly, the foreign material sucked into the inside of the monitor intrudes between a liquid crystal display panel and a backlight through a gap formed in the liquid crystal display panel.
To overcome this drawback, JP-A-5-66405 (patent document 1) discloses a liquid crystal display device which sequentially arranges a backlight and a liquid crystal display element on a printed circuit board, wherein a wall which is hermetically adhered to the liquid crystal display element is arranged outside a display screen of a diffusion plate which constitutes a backlight thus preventing the intrusion of a foreign material into the inside of the liquid crystal display device.